The present invention relates generally to nuclear fuel assemblies and, more particularly, to an apparatus and a method for facilitating scratchless loading of fuel rods into a fuel assembly.
Most nuclear reactors utilize cores composed of elongate, upright fuel assemblies each comprising a unitary structure, or skeleton, and a plurality of fuel rods or pins loaded into the skeleton and supported therein in a predetermined array and in parallel spaced relationship with respect to each other. Usually, support for the fuel rods is provided by transverse grids which form part of the skeleton and are spaced from each other therealong, each such grid being constructed of plates or straps interlaced in an eggcrate-like manner to define open cells through which the individual fuel rods extend. The straps have formed thereon detents which protrude into the respective cells so as to engage the fuel rods therein and to hold them against vibration and against lateral displacement such as could result in localized neutron flux peaking and, consequently, in hot spots. The detents associated with each cell usually comprise springs and dimples formed from the metal of the strap portions defining the walls of the cell, as disclosed in U.S. Pat. Nos. Re. 28,079 to Andrews et al., and 3,920,515 to Ferrari et al., for example, or they may comprise only dimples arranged in sets, as disclosed in copending U.S. patent application, Ser. No. 729,387 of J. A. Rylatt, filed May 1, 1985, and assigned to the present assignee (WE-52,505).
In order that such detents can perform their intended function effectively, they are designed to exert upon the fuel rods a considerable restraining force, such as 8 to 10 lbs. (ca. 3.6 to 4.5 kg). This force, while beneficial on the one hand, poses a problem on the other, namely, one arising during loading of the fuel rods into the skeleton when the fuel rods slide over the detents in moving therepast and thereby might be scratched. Scratches in the outer surfaces of fuel rods tend to induce and aggravate corrosion of the fuel rod cladding during use. Moreover, scratching of fuel rods has been observed to result in a buildup of fine chips scraped from the cladding surface and accumulating in the grid cells, there to form so-called "gall balls" which will fret against, and may eventually even fret through, the cladding of fuel rods extending through the affected grid cells. Moreover, if gall balls collect at cell springs, they can cause abnormal spring deflection lessening the restraining force exerted by the springs upon the fuel rods; and if such gall-ball buildup occurs at springs located in peripheral cells of a support grid, it can result in spring deformation causing the springs abnormally to protrude outward from the peripheral grid straps and beyond the grid boundary. Of course, chips scraped from the cladding of fuel rods and accumulating in grid cells also reduce the free cross-sectional area of the cells and, hence, impair the flow of reactor coolant therethrough.
The problem of fuel rod scratching is well recognized in the art, and various endeavors have been made to overcome it. Thus, U.S. Pat. No. 3,757,403 (Bleiberg) proposes to subject fuel rods, prior to their insertion into the grids of a fuel assembly, to a cooling treatment carried out in a humid atmosphere and in a manner such as to form, on each fuel rod to be inserted, a hoarfrost-like coating intended to act as a lubricant during the insertion of the fuel rod, a function which of course the hoarfrost-like coating can perform only so long as it remains intact and is not removed by the grid-cell detents bearing against the fuel rod as it is being inserted through the cells of successive grids.
Another technique intended to permit scratchless fuel-rod insertion is disclosed in U.S. Pat. Nos. 3,795,040 and 3,892,027 (both to Jabsen) which propose first to insert a rod-like spring retractor of generally square transverse cross-section through axially aligned cells of the grids; then to partially rotate the spring retractor about its longitudinal axis in order to cam fuel-rod supporting detents on resilient wall portions of the grid cells out of the path of the fuel rod to be inserted; thereafter to lock the detent-bearing cell wall portions in their deflected positions by means of bar-shaped keys first inserted laterally into the grid cells through cut-outs in the straps or plates of the respective grids, and then turned to locking positions; thereafter to withdraw the spring retractor from the cells and to insert the fuel rod into them; and finally to turn the keys to unlocking positions and to withdraw them from the grid cells, thereby enabling the detent-bearing cell wall portions to resiliently return to their normal positions and, hence, enabling the detents thereon to engage the fuel rod.
In Japanese Patent Document No. 53-11294, there is disclosed an assembly made of stainless steel and comprising a unitary member which consists of an end block and, extending therefrom, four thin armoring strips which are similar in length to a fuel rod and are cylindrically arrayed in quadrature and in parallel spaced relationship with respect to each other. Prior to insertion of a fuel rod into a fuel assembly, the fuel rod is placed between the thin armoring strips, the distal ends of which are then fixed in position by means of a holding ring placed round the strips and the fuel rod, and a fixing ring applied to the holding ring. Thereafter, this whole assemblage is inserted into the fuel assembly by axially moving it through serially aligned cells of the support grids while, at the same time, carefully maintaining the four armoring strips aligned with and protectively sandwiched between the respective grid-cell detents and the fuel rod cladding. When insertion is complete, the entire assemblage is rotated about its longitudinal axis to an extent calculated to slide the armoring strips laterally from between the fuel rod and the respective detents applying the fuel-rod restraining force, whereupon the holding and fixing rings are removed from the one end of the fuel rod, and the unitary member consisting of the end block and the strips then is ready to be withdrawn from the other end, provided that all of the armoring strips have in fact come free of the detents and still are straight, undistorted and flat against the inserted fuel rod.
In the present assignee's copending first patent application initially cross-referenced herein, there is disclosed an arrangement for facilitating the loading of fuel rods into a fuel assembly, comprising first means insertable axially into the cells of the spacer grids of the fuel assembly so as to deflect the springs therein to retracted positions, and second means insertable laterally into the respective grids so as to hold the deflected springs in their retracted positions upon withdrawal of the first means and during insertion of the fuel rods, the movements of the first and second means being linear. Finally, the present assignee's second patent application initially cross-referenced herein discloses a spring retainer apparatus designed for use in retracting detents in the form of grid springs which are arranged in pairs, with the springs of each pair disposed back-to-back with respect to each other and protruding each into one of two adjoining grid cells.
These earlier techniques, with the exception of the one relying upon the formation of hoarfrost-like deposits as lubricants, require the use and manipulation of several independent parts and elements, and each is designed for use with a particular configuration of fuel-rod detents employed in the support grids. Thus, there exists a need for a different approach, one which is not subject to these limitations, and the present invention has for its principal object to satisfy this need.